Historically, computers communicated over dedicated communication lines or through specific connections such as a modem connection from one computer to another. Computers have also been connected in networks that allow communication among a number of computers. If a computer is added or removed from the network, typically a human operator notifies a dedicated network application of the addition or removal. In another conventional approach, a specialized network administration node tracks the changes to the network.
In a modern network, such as the Internet based on IP (the Internet Protocol), a human operator can add or remove a computer (e.g., a server serving the World Wide Web or web site) from the network. Another computer (e.g., client) is not necessarily aware of the change. It is possible for clients to use monitoring software applications or routines to keep track of the availability of a remote web site by regularly checking the site. For example, the monitoring software application can use a “ping” command to check on the status of a particular server serving a web site.
In many conventional systems, clients are interested in maintaining or understanding pathways that connect with other computers. The status of the pathway may be based on virtual connections, as well as the physical connections between computers. In a network like the Internet, a virtual connection between computers may be viewed as one data communications connection or pathway based on one or more physical connections between the computers. For example, one client computer can make a virtual connection to a server computer by referencing an HTTP (Hypertext Transfer Protocol) hyperlink in one web page to the server. The client and server computers can be located in the same building. Alternatively, the two computers may be located in different cities. In this case, the physical connections between the two computers may pass through many intermediate computers (e.g., many nodes or hops on the Internet) while the virtual connection is viewed as one connection (e.g., between a web site served by the client and the server computer).
A pathway between two computers can be established or interrupted due to the change of a physical connection or due to the change of a virtual connection. In conventional systems of very large networks, such as the Internet, computers are often not aware of a change in the status of other computers connected to the network until they try to access a computer and then discover a change, such as a removal of the computer from the network.